Cyanide recovery process



June 9, 1931.

L. D.. MILLS ETAL GYA'NIDE RECWERY Paocss w Filed Aug. 1Q. 1928 JUN/770W70 W4 TIE,

T0 jnfLTEB NE YS f andcopper.

Patented June 9, 1931 f narran PAT LOUIS D. MILLS AND THOMAS B. CROWE,OF PALO ALTO,v CALIFORNIA, *ASSIGNORS TO THE MERRILL COMPANY, OF SANFRANCISCO, CALIFORNIA, CORPORATION OF oarirronnm f oYAnrnn RECOVERYrinconesY Application led Augustilll, 1928, Serial No.`299,554. i

This invention relates generally 4to processes for the recovery ofcyanogen from cya'- nide solutions used in treatment of ores containinggold, silver and copper.

ln treating such ores according to the cyanide process, a certain amountofv the cyanogen of the'trea'ting solutionis rendered unfit Jfor furtheruse in dissolving metals.`

This cyanogen constitutes what is termed the cyanide consumption of thetreatment process and is caused from the factthat copper, sulphur andiron enter into the solution forming `With the cyanogen, compounds suchas sodium copper sulpl'iocyanide, sodium copper ferrocyanide, and sodiumcopper cyanide. TWhen precipitation With Zinc' 1s employed,

Vthe copper contained in these compounds only partly precipitated,andfrequently the remainderv of the copper, together with the cyanogencompounds With which 1t 1s vassociated, and in addition unprecipitatedgold yand silver are Wasted with Vthe mill tailings. Processeshavepreviously 'been'proposed for the recovery of an available form ofcyanogen froml such mill tailings, and itis a general object ofthis'invention to improve upon such processes.

It is a further object of this invention to `devise a process of thecharacter described above which will be eicient in operation and willmake possible more complete recovery of cyanogen, and of metals such asgold, silver It is a `further object of this invention to devise acyanide recovery process in which a suitable chemical after thesolutionhas been acidiiied.

It is afurther object'of this invention to devise a novel method ofel'ecting purificaation; Y

tion of hydrogen cyanide gas recovered`I by roasting of'cyanogencompounds in a vreducing atmosphere.

It is a further'object of thisY invention to effect more completerecovery ofcyanogen from compounds,precipitated from cyanide Furtherobjects of the inventionv Will appear from the following description inwhich We have set forth the preferred embodiment of vour invention. ltisto be understood that vthe appended claims' arey to befaccorded arange of 'equivalents consistentk With the state Vof the prior art. A

Referring to the drawing:

'llhey single vievv is a` iioW, sheet representing diagrammatically thesteps of our process accordingto the preferred embodimentof heinvention.

Our process can be -brieily'outlined as follows: The protected cyanideysolution to be treated is acidifiedftoeii'ect precipitation ofcertainfmetallicy compounds and to cause formation of' hydrogen cyanide.Hydrogen vcyanide is Vthen dispersedfronithe solutiom'and isabsorbed ina suitable alkali. The dispersed solution istreated With `a suitablechemical, preferably copper sulphate, to effect further precipitationvof` metalli'cicompounds. The precipitate vis lremoved yfrom the solutionandv is digested'4 with suitable chemicals, such as acid-and coppersulphate,

cyanogen in a subsequent roasting'operation.

lmpurehydrogen cyanide gas produced byk stood by reference to theaccompanying drawing. l At the beginning ofthe Vcycle it is presumedthat the solution to be 'decyanided, vwhich is protected vvitha suitablealkali, is

in the storage tank 1. VFrom this tank the "'15 to make possible morecomplete recovery' of f solution passes thru anacidilier 2, Where theysolution is rendered acid by a suitable chemical, preferably asulphur-oxygen acid. Good resultsy havevbeen obtained by the use ofsulphur dioxide gas, and `preferably. Vthe acidiyfier 2 causes thesulphur dioxide to beintimately contacted with the cyanide solution.

According to Well knownreactions, acidification of such cyanidesolutions causes release of hydrogencyanide and precipitation of certaincompounds of metals.' Y v` `After 'acidicatiorn the solution and anyprecipitate which may have formed areintro- 'Y :in

duced into a disperser 8, Where the hydrogen cyanide is removed bycontacting the solution With large volumes of a suitable fluid such asair. In practice We prefer to connect disperser 3 in series With anabsorber t and to circulate large volumes-of air through the same bymeans of fan 5. Air is passed thru disperser 3, is brought incounterflovv contact With the cyanidesolution, and is then passed thruabsorber 4 Where the air and hydrogen cyanide are brought incounterfioiv contact with a suitable alkaline solution, such as calciumhydroxide or lime Water. From absorber 4 the air is reintroduced intofan 5. The hydrogen cyanide reacts With the lime Water to form'calciumcyanide, in Which form the cyanogen is available for reuse in treatingores. The solution and precipitate remaining in disperser 3 is thenintroduced in a reservoir 6 provided With suitable agitating means. Inreservoir 6 We add to the solutionachemical capable of effecting furtherprecipitation of compounds of gold, silver and copper.

Our process is particularly adapted for use with cyanide solutionsemployed in theV treatment of ores containing a fairly high percentageof copper, sulphur and iron. Upon acidification of solutions employed inthe treatment of such ores, certain soluble cyanogen containingcompounds of metals are present which.v are not precipitated. If suchcompounds are Vpermitted-to remain in solution,

"they forma vsource' of Waste cyanogen and met-alsV and therefore reducethe efficiency of the recovery. process. VThe introduction of a suitablechemical into the agitator 6 is for the purpose yof breaking vdown suchsoluble compounds of .'-metals and to precipitate compounds from Whichcyanogen and metals can be subsequently removed.r To effect this furtherprecipitation, We prefer to employ copperjsulphate, although a varietyof salts of metals may be employed. In addition to copper sulphate goodresults can be obtained by employingk chlorides of copper, or salts ofmetals such as silver and mercury which are V`Vsoluble in the acidsolution. Inany event the salt or `other chemical employed should beeffective to combine With sulphocyanides and ferro cyanides to forminsoluble compounds of metals which precipitate from the solution. Thecopper sulphate introduced at this point may be miXed With a certainamount of iron salts, as for example ferrous' sulphate, in which eventboth salts enter into the reaction. Copper saltsserve to precipitateboth rsulpho and ferro cyanides While an iron salt is useful in that itaids in the precipitation offerro cyanides, When such compounds arepresent. A mixture of copper sulphate and ferrous sulphate is obtainedin an other step of our' process 'as will be presently explained. Inpractice a slight excess `of copper salt is employed. to render theprecipitation complete., The salt can be added before the solutionenters the disperser or Within the disperser, but We prefer that it beadded in a separate agitating vessel subsequent to dispersion.

From the agitator 6, the mixture is introduced into a filter 7, Wherethe solution is separated from the precipitate and is discharged toWaste, and the precipitate removed in the filter is then introduced intothe digester 8. The precipitate novv contains substantially all of thecyanogen present in the original cyanide solution, With the exception ofthat removed in the disperser, and the copper present is principally inthe form of copper sulphocyanide or copper ferrocyanide or both. Inaddition the precipitate contains practically all of the metals such asgold or silver, or both, Which Were originally present in the cyanidesolution being treated.

rIhe digester 8 is for the purpose of chemically treating theprecipitate in order to effect certain changes making possible morecomplete recovering of cyanogen in the subsequent roasting operation.This step is not essential to the process and may be omitted in case theadditional recovery effectedis not a material factor in efficiency ofthe process, but it is to be preferred in case the cyanide solution isof such a character that the precipitate recovered' from the filter 7contains a substantial percentage of compounds such as copperferrocyanide, from Which a complete recovery cannot be effected byroasting in a reducing atmosphere. The treatment of the precipitate indigester 8 serves principally Vthe function of converting copperferrocyanide and iron ferrocyanide to copper cyanide, in addition toWhich iron sulphate 4 and sulphuric acid are formed. As a suitablechemical to effect this action, We prefer to employ a. salt of a metal,such as copper sulphate, and this material is introduced int-othedigester 8 and agitated together With the precipitate at an elevatedtemperature.

Heating of the contents of the digester can be conveniently accomplishedby supplying` steam as indicated. Although the reaction of coppersulphate with metal ferrocyanides results in the'formation of sulphuricacid, We prefer to further acidity the mixture by the addition of asuitable acid, preferably a sul'- phur-oxygen acid. For this purposeWater is suppliedto the contents of digester 8 and the mixture iscontacted With sulphur dioxide as indicated. In practice the coppersulphate can also be mixed With other salts such as ferro-us sulphate.

The resulting mixture obtained in the digester 8 is passed thru thefilter 9, and the separated precipitate is introduced into a roaster 10.In the roaster a batch of precipitate is first subjected at an elevatedtemperature to a reducing gas containing hydrogen, as for exampleproducer gas. We have secured good results in practice by employingroasting temperatures from 600 to 900?J Fahrenheit. As a result of thistreatment cyanogen containing compounds are' broken down to formhydrogen cyanide gas, which however is in-an impure state as it is mixedWith other gases, principally hydrogen sulphide.

-nide and the hydrogen sulphide are absorbed bythe alkalinecyanidesolution and the additional hydrogen cyanide sointroduced is ulti-.mately recovered in available form from the absorberl 4. Contrary toWhat might ordinarily be expected, We have discovered that theintroduction of hydrogensulphide at the vbeginning of the cycle doesnotmaterially increase the amount of precipitate recovered from thedisperser 3, and apparently it forms soluble compounds which pass. offtogether Withthe Waste solutionfrom filter 7 After comparativelycomplete reduction of the contents of'roaster 10 the iiitroductionjof areducing gas is discontinued and an oxidizing gas, such as air, isintroduced. jThe metal sulphides formed during the reducing oper-ationarethen converted-to oxides, and sulphur dioxide is evolved. In practiceWe prefer to utilize this sulphur Adioxide by employing it for aidingacidification of the cyanide solution precipitated "in disperser 3.Therefore Wehave indicated introduction of this sulphur dioxide gas fromthe roaster 10 kin to absorber 11. While introduction of sulphur dioxideinto the absorber' 11l aids in effecting acidification, it is not ofitself suffi tains oxides of the metals in the precipitate,y

chiefly copper together With goldand silver. This calcine is preferablyintroduced' into a digester 13, for the purpose of producing Asuitablemetal salts for use in agitator Gand digester 8. For example indigesterk 13 We introduce the acidsolution recovered from vthe filter 9,which is av solutionl of irony sull -phate and sulphuric acid. Copperand/or iron oxides inthe c'alcine are thereby convertedto sulphatesofthese metals, and the solution ofthese soluble salts is separatedAfrom the oxides of the remaining metals by means `of filter 14and'is'then utilizedy for introduction into agitator 6 .and digester 8as indicated. Thek f undigested metals recovered from filter 14,consistingprincipally of oxides of copper, together `With gold andsilver, are

:removed Afrom filter 14 and smelted to obtain the desired metals.V

It Will .be noted that the above. process not only effects more completerecovery of cyanoe gen from cyanide solutions, but the entire recoveryprocess is carriedV out by the use of a minimum amount of' additionalchemical.

The process also makes possiblea relatively simple apparatus,particularly since We dispense with the use of additional means for fromthe roaster 10.

ln order to yfurther clarify our invention, the following example. isgiven of our process as Vapplied to .cyanide solution einployedfin thetreatment of a `certain ore; It is to be Vunderstood that'this exampleisfor the purpose of description only and is not to limit the inventiondefined by the appended claims.

A base ore containing approximately forty per cent sulphur as sulphidesof iron, copper, Zinc and lead, was crushed and ground in water andconcentrated by the flotation process.y Theftailings, afterconcentration, con-k tained..055 oz. gold, .87 oz. silver, .16 per centcopper, 1.16 per cent Zinc and .112 per cent `7.5 purifying the hydrogencyanide gas recovered lead. 4000 grams of these tailings were treatedwith protected alkalinecyanidek solution containing 11.25 gramsof sodiumcyanide.r The resulting solution contained 4.7 4 grams 1.

of free-sodium cyanide. The remainderof y'the cyanogen originallypresent, equivalent to '6.51 grams of sodium cyanide, was present..

in the solution mostly'a's metal sulphocyajnides and metal ferrocyanidesWith smaller amounts of metal cyanides. v

VUpon acidifying the solution With sulphur dioxide and adding4 grams ofcopper inthe form of coppersulphate, and upon' recovering the liberatedhydrogen cyanide,the recovery fromthe absorber 4 Was found equivalent to7.88 grams of sodium cyanide. The precipitates of copper ferrocyanide`and copper vsulphocyanide Were formed into a pulp with Vvvater anddigested With steam, sulphur di- .fi oxide and copper sulphate,`.andafter filtration, the resulting cop-per-sulphocyanide and copper cyanidewas heated in contact vvith ,n villuminating: gas. l The. evolvedhydrogen cyanide and hydrogen sulphideivvas absorbedf in a solutionofcaustic soda Which Was subsequently acidified with sulphur dioxideland treated to remove the pure hydrogen cyanide `which, afterabsorption in fresh caustic soda solution, vvas found to be'equivalentto 1.49

grams of sodium cyanide. Thus the total recovery or regeneration by theuse of the proc- 937 gramsffrom anoriginal'1125 grains leaving va essvvas17.88grams 1.49 grams consumption of 1.88.grams, orl .'94 poundssodium cyanide per ton of tailings treated. i

The tailings after cyanide treatment con- 4 tained .015"oz.` gold and.32 oz, silver, showing that 72.7 per cent off-the gold and 63.2 per`cent of the silver hadv been dissolved bythe cyanide solution. The acidiiltrate from the filter 7 contained only tracesof gold and sil- Yver'showing that substantially all of the gold and silver dissolved fromthe Vtailings was contained inthe residue from the filter' lll.

. It will be noted in the above example that precipitation of metalsfrom the solution Was effectedV Withinour recovery process Withoutpreliminary precipitation With zinc dust. This method of operation isdesirable Where the ore is of Ylovv grade.v Our process is alsoapplicableto the recovery of cyanide from protected solutions from Whichmetals have been partially precipitated, as by treatment With zinc. f

We claim:

1. In a process for the recovery ofc-yanide 'from cyanide solutionemployed in the treatment of ore, the steps of roasting metallic cyanidecompounds precipitated from a cyanide solution', introducing hydrogencyanide recovered from said roasting operation into vanother cyanidesolution, and recovering purified hydrogen cyanide from said lastmentioned solution.

2. In a'process for the recovery of cyanide from cyanide solutionemployed in the treatment of ore, the steps of roasting metallic cyanidecompounds precipitated from a cyanide solution, introducing unpurifiedhydrogen cyanide recovered from said roasting operation into anothercyanide solution, and recovering purified hydrogen cyanide from saidlast mentioned solution.

3. In a process for the recovery of cyanide from cyanide solutionemployed in the treatment of ore, the step of roasting metallic `cyanidecompounds precipitated from a cyanide solution, introducing unpurifiedhydrogen cyanide recovered from said roasting operation into a cyanidesolution from which substantiallyno cyanide has heen removed, andrecovering hydrogen cyanide fro-in said 4last mentioned solution.

4. In a process for the recovering of cyanide from cyanide solutionemployed in the treatment of' ore,-the step of roasting metallic cyanidecompounds precipitated from a cyanide solution, introducing unpurifiedhydrocyanic acid recovered fromfsaid roasting operation into. anunacidiied cyanide solution, and recovering hydrogen cyanide from saidlast mentioned solution.

5. In a process for the recovery vof cyanide from cyanide solutionemployed in theftreatment of ore, the step of roasting metallic cyanidecompounds precipitated from a cyanide solution, said roasting beingeifected in the presence of a. reducing gas, introducing unpurifiedhydrogen cyanide gas recovered from said roasting operation into anunacidifie-d cyanide solution, acidifying said last named' solution, andremovinghydrogen cyanide Ltrom said last mentioned solution.

r 6. In a process for the recovery of cyanide from cyanide solutionemployed in the treatment of ore, the steps of roasting metallic cyanidecompounds precipitated from a cyanide solution, introducing unpurifiedhydrogen cyanide in the form in Which it is recovered from said roastingoperation into an unacidified cyanide solution, eecting precipitation ofmetallic compounds from said last mentioned solutions 'by acidificationand by the addition of a. salt of a heavy metal, and removing purifiedhydrogenl cyanide from said Vlast mentioned solution.

7. In a process for the recovery of cyanide from cyanide solutionemployed in the treatment of Ore, the steps of roasting metallic cyanidecompounds precipitated from a cyanide solution, introducing unpurifiedhydrogen cyanide recovered from said roasting operation into anunacidified cyanide solution,

eifec-ting Aprecipitation of metallic compounds from said last namedsolution by acidiiication and by the addition of a copper salt, anddispersing puried hydrogen cyanide gas from said solution by contact ofthe solu-A tion with a carrier gas.

8. In a cyclic process for the recovery of cyanide from cyanide solutionemployed in the treatment of' ore, the steps of roasting metalliccyanide compounds precipitated from a cyanide solution, introducingunpu- Yrified hydrogen cyanide recovered from said metallic cyanidecompounds precipitated` from a cyanide solution, introducing unpurifiedhydrogen cyanide recovered from said roasting operations into anothercyanide solution, effecting precipitation of said other solution by acidand by addition of a salt of a heavy metal, removing` freed hydrogencyanide from said other solution, and utilizing compounds precipitatedfrom said other solution for step one of the process.

10. In a cyclic process of the character described, the steps ofprecipitating metallic compounds from a cyanide solution by acidirfyingwith an oxide of sulphur, roasting the LOUIS D. MILLS. THOMAS B. CROW?.

'set our lhands'.

-llO

